As optical recording medium for recording a variety of information such as audio information and video information, various types of optical recording mediums are available in the form of a read-only optical recording medium such as a CD and a CD-ROM, a rewritable optical recording medium such as a magneto-optical disk and a phase-change optical disc and a write-once optical disc such as a CD-R made of an organic material.
Information recording layers comprising these optical recording mediums are indented to form thereon very small indentations such as phase pits and pregrooves to record data information, a tracking servo signal and the like.
Recently, there is an increasing demand for realizing optical recording medium capable of recording a larger amount of information, and a DVD (Digital Versatile Disc) of a two-layer structure having laminated first and second information recording layers, for example, becomes commercially available.
FIG. 4 is a schematic cross-sectional view illustrating an optical recording medium 200 of a two-layer structure in which a first information recording layer 231 and a second information recording layer 232 are laminated to each other.
In the first information recording layer 231, a translucent reflective film 223 made of a suitable material, such as Au, Si, AgPdCu and AgPdTi, is deposited on first very small indentations 211 which are formed at the same time a first substrate 201 is molded by injection molding.
In the second information recording layer 232, a reflective film 224 composed of a suitable thin film such as an Al thin film and an Al alloy thin film, is deposited on second very small indentations 222 which are formed at the same time a second substrate 202 is molded by injection molding.
A transparent adhesive layer 203 laminates the first and second substrates 201 and 202 with the first and second information recording layers 231 and 232 being facing to each other to form a two-layer information recording layer.
It is desirable that a set of optical heads should be used to reproduce or record or to reproduce and record (hereinafter simply referred to as “reproduce or record”) information from the first and second information recording layers 231 and 232 of the optical recording medium 200 with irradiation of laser beams from the same side of the optical recording medium, e.g., from the first substrate 201 side, for example, so that a drive apparatus therefor can be simplified, the optical heads can access these information recording layers 231 and 232 in a short time and can continuously record or reproduce these information recording layers.
When the optical head irradiates laser beams on the first and second information recording layers 231 and 232 from the same side of the optical recording medium to record or reproduce or record and reproduce the first and second information recording layers, the same optical head focuses a laser beam L on the first information recording layer 231 to record or reproduce or record and reproduce the first information recording layer 231 as shown by a solid line in FIG. 4 and the same optical head focuses a laser beam L on the second information recording layer 232 to record or reproduce or record and reproduce the second information recording layer 232 as shown by a dotted line in FIG. 4.
In order that the same optical head may record or reproduce or record and reproduce the first and second information recording layers 231 and 232 with irradiation of laser beams, the first information recording layer 231 has the translucent reflective film 223 formed at its entrance side for inward laser beams to reflect part of irradiated laser beams so that the first information recording layer 231 may be recorded or reproduced and to pass part of laser beams to allow part of laser beams to travel to the second information recording layer 232 so that the second information recording layer 232 may be recorded or reproduced.
The transparent adhesive layer 203, made of an adhesive material having a high transmittance with respect to laser beams, laminates the first and second information recording layers 231 and 232 with a distance long enough to prevent their reproduced signal from interfering with each other. Therefore, the optical heads can adjust objective lenses so as to properly focus the laser beams on the positions corresponding to the respective information recording layers 231 and 232, thereby making it possible to reproduce information from the respective information recording layers with high accuracy.
Design of films of the translucent reflective film 223 of the first information recording layer becomes extremely important to realize the above-mentioned signal reproducing method.
Au, Si, AgPdCu and AgPdTi are used as materials for forming the translucent reflective film 223 as described above.
So far these materials have been used as the materials of the translucent reflective film because they can satisfy optical characteristics of the translucent reflective film 223 from a standpoint of reflectance and transmittance relative to laser beams and they can be easily deposited as thin films by sputtering.
However, Au encounters with a problem in which costs of materials increase. Although Si is relatively inexpensive, it is poor in adhesive property with which it is bonded to the adhesive material comprising the transparent adhesive agent layer 203 or it is bonded to the material of the substrate 201. Accordingly, silicon is not sufficiently reliable in mechanical deformation such as bending or warping or under severe circumstances with high humidity.
Further, when the Si film is compared with metal thin films, Si is easily separated from the inside of a sputtering chamber where it is stuck in the sputtering process when the film is deposited, i.e., so-called particles are easily produced so that an error rate is caused to be degraded.
The translucent reflective film needs a film thickness ranging from 5[nm] to 25[nm] when it is made of metals or Si semiconductor materials and so on that have been so far used to form ordinary translucent reflective films. This film thickness of the translucent reflective film is thin as compared with a film thickness ranging from 35[mm] to 60[nm] of a reflective film of an ordinary compact disc, for example. The metals or the Si semiconductor materials and the like for use in the ordinary translucent reflective film are easily oxidized at their surface by influences exerted from the substrate 201 side after they have been stored for a long time and further after they have been stored under circumstances with high temperature and high humidity. Accordingly, oxidation produced on the surface changes a reflectance of the translucent reflective film having such film thickness considerably, and this translucent reflective film is poor in weather resistance.
The translucent reflective film 223 is damaged not only by the influence from the substrate 201 side but also by oxidation from a contact portion where it is brought in contact with the transparent adhesive agent layer 203. The oxidation from the contact portion changes the reflectance of the translucent reflective film and deteriorates the jitter of the reproduced signal unavoidably.
Apart from the problem of the above-mentioned oxidation, the translucent reflective film encounters with a phenomenon in which atoms are caused to move within the deposited film to increase thermal conductivity or reflectance of the translucent reflective film after the translucent reflective film has been left under circumstances with high temperature where so-called annealed effect happens. This phenomenon becomes a serious problem depending upon compositions of the translucent reflective film.
The optical recording medium having the multilayer structure have various problems that should be solved when to make its translucent reflective film. To improve the weather resistance of the translucent reflective film and to reduce the cost thereof are important problems that should be solved when to make the optical recording medium having the multilayer structure commercially available on the market.
Weather resistance of the reflective film is important not only in the reflective film of the optical recording medium having the multilayer structure but also in the reflective films for use in rewritable optical recording mediums such as a magneto-optical recording medium, a phase-change optical recording medium and a dye-system optical recording medium in which various material films are laminated as information recording layers. When the reflective film is degraded due to aged deterioration such as oxidation, the deteriorated reflective film changes not only quality of the reproduced signal but also recording conditions such as recording sensitivity.
The AgPdCu thin film or the AgPdTi thin film is inexpensive as compared with a simple substance of Au from a money standpoint and cannot be separated from the adhesive material or the material of the substrate unlike the Si film.
Recently, a demand for higher recording density or larger recording capacity is increasing, and laser light with a short wavelength is used as laser light for use in recording or reproducing information, whereby surface recording density can be increased. In this case, recording pits should be formed with higher accuracy, and even though the amount in which the jitter in the reproduced signal is deteriorated is small to the extent that it is allowable when the surface recording density is low, such very small amount of the deteriorated jitter raises a serious problem as the surface recording density increases.
Further, as the information recording layer is formed as the information recording layer having the two-layer structure as described above or the information recording layer is formed as an information recording layer having a multilayer structure of more than two layers, the translucent reflective film in each information recording layer at the entrance end side of inward light is progressively decreasing its film thickness more because reflectance and transmittance of each layer and the like should be selected properly. Therefore, this translucent reflective film needs higher weather resistance.
Specifically, this translucent reflective film needs high weather resistance to the extent that reflectance can be prevented from being changed and that the jitter in the reproduced signal can be prevented from being deteriorated under severe circumstances for a longer time.
Furthermore, a demand for manufacturing optical recording mediums more inexpensively also is increasing more than before.
An object of the present invention is to provide an optical recording medium in which weather resistance of a translucent reflective film and a reflective film comprising information recording layers of an optical recording medium, in particular, weather resistance of the translucent reflective film with a special characteristic such as translucency can be improved and in which a cost thereof can be reduced.